A wide spectrum of microelectronic and microelectromechanical systems (MEMS) applications has increased the need for lower-temperature, thermally decomposable sacrificial materials. This includes fabrication of air-gaps in electrical interconnects, MEMS, microfluidic devices, and micro-reactors.
The formation of air-gaps is important in electrical interconnects because it lowers the effective dielectric constant of the matrix. The fabrication of buried air channels is useful for the creation of vias in multi-level wiring boards, micro-display boards with high resolution, and ink-jet printer heads. In MEMS technology, the fabrication of micro-air cavities may alleviate the stress associated with thermal expansion of materials and also can act as a temperature-activated release material.
Microfluidic devices and microreactors, fabricated with air-gap technology can be used for miniature-scale chemical syntheses, medical diagnostics, and micro-chemical analysis and sensors. In such devices, liquids and gases are manipulated in microchannels with cross-sectional dimensions on the order of tens to hundreds of micrometers. Processing in such microchannel devices offers a number of advantages including low reagent and analyte consumption, highly compact and portable systems, fast processing times, and the potential for disposable systems.
In spite of all of their promise, however, microfluidic devices are currently being used in a limited number of applications and are in general still rather simple devices in terms of their operational complexity and capabilities. For example, in terms of making truly portable microanalytical systems, one of the current difficulties involves the simple integration of electronic (e.g., sensing methods) and fluidic elements into the same device. One of the most important issues, controlling this ability to integrate functions into the same device, and thus controlling the level of functionality of a microfluidic device, is the method used to fabricate the structure.
The applications for a microfluidic device require the formation of buried microchannels in several different materials at a variety of temperatures. Polycarbonates have been used as a sacrificial material in fabricating nanofluidic devices by electron beam lithography. C. K. Harnett, et al., J Vac. Sci. Technol. B., vol. 19(6), p. 2842, 2001. Air-gaps have been also fabricated using the hot-filament chemical vapor deposition of polyoxymethylene as a sacrificial layer. L. S. Lee, et al., Electrochem. and Solid State Lett., vol. 4, p. G81, 2001. Further, highly structured, dendritic material, specifically hyperbranched polymers, have been used as a dry-release sacrificial material in the fabrication of a cantilever beam. H-J. Suh, et al., J. Microelectromech. Syst., Vol. 9(2), pp. 198-205, 2000. Previous work has also fabricated air-gaps using non-photosensitive sacrificial polymers that decompose in the range 250-425° C. P. A. Kohl, et al., Electrochemical and Solid State Lett., vol.1, p.49, 1998; D. Bhusari, et al., J Micromech. Microeng., vol.10(3), p. 400, 2001.
FIGS. 1A-1H are cross-sectional views that illustrate a previously proposed method 100 for forming a buried air cavity using a non-photosensitive sacrificial material. FIG. 1A illustrates a substrate 10, prior to having a non-photosensitive sacrificial material 12 disposed thereon by, for example, spin-coating as in FIG. 1B. FIG. 1C illustrates a hard mask 14 disposed on the sacrificial material 12. FIG. 1D illustrates a photolithographed and etched mask portion 16 disposed on the hard mask 14, while FIG. 1E illustrates the removal of the mask portion 16 and portions of the sacrificial polymer material 12 exposed to the plasma etching. The hard mask 14 is then removed, as shown in FIG. 1F.
FIG. 1G illustrates the formation of the overcoat layer 18 onto the sacrificial polymer 12 and the substrate 10. FIG. 1H illustrates the decomposition of the sacrificial polymer 12 to form air-regions 20. The sacrificial polymer 12 has conventionally been decomposed by heating the sacrificial polymer 12 to a temperature sufficient to decompose the polymer (e.g., about 300-425° C.). Prior sacrificial polymers have required high decomposition temperatures (e.g., about 300-425° C.), which limits the types of overcoat materials and substrates that may be used, as the overcoat and substrate materials must be able to withstand the high temperatures needed to decompose the sacrificial polymers. Even the use of a polycarbonate polymer as the sacrificial polymer lowers the decomposition temperature to about 250-280° C.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.